Heart failure, predominantly caused by myocardial infarction, is the leading cause of death in both adults and children worldwide and is increasing exponentially worldwide (Bui, A. L. et al. (2011) Nat. Rev. Cardiol. 8:30-41). The disease is primarily driven by the loss of ventricular muscle that occurs during myocardial injury (Lin, Z. and Pu, W. T. (2014) Sci. Transl. Med. 6:239rv1) and is compounded by the negligible ability of the adult heart to mount a regenerative response (Bergmann, O. et al. (2009) Science 324:98-102; Senyo, S. E. et al. (2013) Nature 493:433-436). Although heart transplantation can be curative, the markedly limited availability of human heart organ donors has led to a widespread unmet clinical need for a renewable source of pure, mature and functional human ventricular muscle tissue (Segers, V. F. M. and Lee, R. J. (2008) Nature 451:937-942; Spater, D. et al. (2014) Development 141:4418-4431).
Human pluripotent stem cells (hPSCs) offer the potential to generate large numbers of functional cardiomyocytes for potential clinical restoration of function in damaged or diseased hearts. Transplantation of stem cells into the heart to improve cardiac function and/or to enrich and regenerate damaged myocardium has been proposed (see e.g., U.S. Patent Publication 20040180043). Combination therapy, in which adult stem cells are administered in combination with treatment with growth factor proteins has also been proposed (see e.g., U.S. Patent Publication 20050214260).
While cell transplantation into the heart offers a promising approach for improving cardiac function and regenerating heart tissue, the question of what type(s) of cells to transplant has been the subject of much investigation. Types of cells investigated for use in regenerating cardiac tissue include bone marrow cells (see e.g., Orlic, D. et al. (2001) Nature 410:701-705; Stamm, C. et al. (2003) Lancet 361:45-46; Perin, E. C. et al. (2003) Circulation 107:2294-2302), adult stem cells (see e.g., Jackson, K. A. et al. (2001) J. Clin. Invest. 107:1395-1402), bone marrow-derived mesenchymal stem cells (see e.g., Barbash, I. M. et al. (2003) Circulation 108:863; Pettinger, M. F. and Martin, B. J. (2003) Circ. Res. 95:9-20), bone marrow stromal cells (Bittira, B. et al. (2003) Eur. J. Cardiothorac. Surg. 24:393-398), a combination of mesenchymal stem cells and fetal cardiomyocytes (see e.g., Min, J. Y. et al. (2002) Ann. Thorac. Surg. 74:1568-1575) and a combination of bone marrow-derived mononuclear cells and bone marrow-derived mesenchymal stem cells (see e.g., U.S. Patent Publication 20080038229). Dedifferentiation of adult mammalian cardiomyocytes in vitro to generate cardiac stem cells for transplantation has also been proposed (see e.g., U.S. Patent Publication 20100093089).
A significant advancement in the approach of cell transplantation to improve cardiac function and regenerate heart tissue was the identification and isolation of a family of multipotent cardiac progenitor cells that are capable of giving rise to cardiac myocytes, cardiac smooth muscle and cardiac endothelial cells (Cal, C. L. et al. (2003) Dev. Cell. 5:877-889; Moretti, A. et al. (2006) Cell 127:1151-1165; Bu, L. et al. (2009) Nature 460:113-117; U.S. Patent Publication 20060246446). These cardiac progenitor cells are characterized by the expression of the LIM homeodomain transcription factor Islet 1 (Isl1) and thus are referred to as Isl1+ cardiac progenitor cells. (Ibid). In contrast, Isl1 is not expressed in differentiated cardiac cells. Additional markers of the Isl1+ cardiac progenitor cells that arise later in differentiation than Isl1 have been described and include Nkx2.5 and flk1 (see e.g., U.S. Patent Publication 20100166714).
The renewal and differentiation of Isl1+ cardiac progenitor cells has been shown to be regulated by a Wnt/beta-catenin signaling pathway (see e.g., Qyang, Y. et al. (2007) Cell Stem Cell. 1:165-179; Kwon, C. et al. (2007) Proc. Natl. Acad. Sci. USA 104:10894-10899). This has led to the development of methods to induce a pluripotent stem cell to enter the Isl1+ lineage and for expansion of the Isl1+ population through modulation of Wnt signaling (see e.g., Lian, X. et al. (2012) Proc. Natl. Acad. Sci. USA 109:E1848-57; Lian, X. et al. (2013) Nat. Protoc. 8:162-175; U.S. Patent Publication 20110033430; U.S. Patent Publication 20130189785).
While human pluripotent stem cells hold great promise, a significant challenge has been the ability to move from simply differentiation of diverse cardiac cells to forming a larger scale pure 3D ventricular muscle tissue in vivo, which ultimately requires vascularization, assembly and alignment of an extracellular matrix, and maturation. Toward that end, a diverse population of cardiac cells (atrial, ventricular, pacemaker) has been coupled with artificial and decellurized matrices (Masumoto, H. et al. (2014) Sci. Rep. 4:5716; Ott, H. C. et al. (2008) Nat. Med. 14:213-221; Schaaf, S. et al. (2011) PLoS One 6:e26397), vascular cells and conduits (Tulloch, N. L. et al. (2011) Circ. Res. 109:47-59) and cocktails of microRNAs (Gama-Garvalho, M. et al. (2014) Cells 3:996-1026) have been studies, but the goal remains elusive.
While the identification of Isl1 as a marker of cardiac progenitor cells was a significant advance, since Isl1 is an intracellular protein it is not a suitable marker for use in isolating large quantities of viable cells. Rather, a cell surface marker(s) is still needed. Furthermore, Isl1 as a marker identifies a population that can differentiate into multiple cell types within the cardiac lineage, and thus there is still a need for markers that identify cardiac progenitor cells that are biased toward a particular cell type within the cardiac lineage, in particular for progenitor cells that differentiate into ventricular cells. Accordingly, there is still a great need in the art for additional markers of cardiac progenitor cells, in particular cell-surface markers of cardiac progenitor cells, that predominantly give rise to cardiomyocytes and that would allow for rapid isolation and large scale expansion of cardiomyogenic progenitor cells. Furthermore, there is still a great need in the art for methods and compositions for isolating cardiac ventricular progenitors, which differentiate into ventricular muscle cells in vivo, thereby allowing for transplantation of ventricular progenitors or ventricular muscle cells in vivo to enhance cardiac function.